Add cpu feature detection for arm/linux

For other platforms where we don't have arm cpu feature detection,
this relies on using the HAVE_NEON define at compile time.

For iOS, this isn't an issue because all iOS armv7 devices have
neon, but for android/linux there are armv7 devices that do not
have NEON, and thus need to have it enabled only at runtime.
Call WelsCPUFeatureDetect within ARM_ASM as well
Hook up building arm assembly in the generic arch and android makefiles